In the field of crude oil expoloitation permanent layer-separating devices without a releasing device are known; these devices can be removed from the casing only by destruction.
Some of these devices can be anchored by means of a so-called "hydraulical landing device" forming a separate assembly unit; in this case efficient setting can be achieved only when beside hydraulic forces a mechanical pulling force is also effective, involving the disadvantageous displacement of the tubing.
As far as we know, the most significant permanent layer-separating devices which can be removed from the casing by destruction, have been produced in USA (Baker -- F,FA,D,DA types; Otis-PERMA-DRILL, as well as the types PD-1,HSP-1 produced by the company CAMCO).
A common feature of the devices enumerated above lies in that for taking the alternating, bidirectional hydraulical and mechanical loads during operation, beneath and above the rubber sealing there is a double-rowed key system and removal from the casing can be performed by the complete destruction of the device only, i.e. by destruction of a key system.
The drawback of such devices lies in that during cutting operation the equipment unscrews and after having removed the upper keyrow, it often slips, thus retrieval becomes difficult and in several cases can be performed by only by a chancy fishing job.